As a common measure against decrease in strength and abrasion resistance that solid materials may experience when used in some environments, a hardened surface layer is formed in the solid materials. The depth of such hardened surface layer is a parameter relying on which the mechanical qualities, such as strength and abrasion resistant, of pieces made from those solid materials are evaluated.
The Vickers hardness test, which is a destructive test, is one of the conventionally-used methods for measuring the hardened surface layer depth. The Vickers hardness test, which cuts a to-be-inspected piece to measure the depth of a hardened surface layer, has drawbacks that it takes time and that it cannot be applied to a piece of a solid material intended to be actually used to form a product. Hence, a non-destructive method for measuring the hardened surface layer depth is demanded.
There is known a non-destructive test method using an ultrasonic wave, in which an ultrasonic wave is transmitted to propagate along the surface of a to-be-inspected piece and the depth of a hardened surface layer is measured relying on the rate of change of sound wave speed (patent document 1).